Transmitter and receiver for and method of transmitting and receiving symbols over an orthogonal time frequency space communication channel subject to doppler spread and multipath

Abstract

Pilot symbols and data symbols of a communication frame for an OTFS transmission system are two-dimensionally arranged along the points of a grid in the delay-Doppler domain. The pilot symbols are surrounded by guard symbols. The number of guard symbols in each direction of the Doppler domain is twice the number of the basis expansion modelling (BEM) basis functions used for modelling the communication channel in a receiver, and twice the maximum time delay in terms of delay bins in each direction of the delay domain. The receiver performs an initial pilot-aided channel estimation using BEM of a first BEM order and using the pilot signals, followed by an initial estimation of data symbols using the initial channel estimation, and iteratively performs data aided channel estimation using BEM of a second BEM order and at least the received data signals, until a termination criterion is met.

The invention claimed is: 
     
         1 . A two-dimensional arrangement of pilot symbols and data symbols of a communication frame for an Orthogonal Time Frequency Space (OTFS) transmission system, the pilot symbols and data symbols being arranged along points of a grid in a delay-Doppler domain, wherein the pilot symbols are surrounded by guard symbols, wherein the number of guard symbols in each direction of the delay-Doppler domain is twice an order of a basis expansion model (BEM) basis functions used for modelling a communication channel in a receiver. 
     
     
         2 . The two-dimensional arrangement of symbols in the communication frame for an OTFS transmission system of  claim 1 , wherein the number of guard symbols surrounding the pilot symbol in each direction of the delay-Doppler domain is twice a maximum time delay in terms of delay bins. 
     
     
         3 . A transmitter of an OTFS transmission system comprising a signal mapper arranged upstream of a first transmitter-side transformation unit and a second transmitter-side transformation unit,
 wherein the signal mapper is adapted to receive a binary data sequence and to output a two-dimensional communication frame (x[k, l]) in the delay-Doppler domain in which pilot symbols, data symbols and guard symbols are two-dimensionally arranged in accordance with  claim 1 ,   wherein the first transmitter-side transformation unit is adapted to receive, at an input, the two-dimensional communication frame in the delay-Doppler domain that is output from the signal mapper, and to output a two-dimensional arrangement of information symbols in a time-frequency domain, and   wherein the output of the first transmitter-side transformation unit is provided to an input of the second transmitter-side transformation unit, which is adapted to output a continuous time-domain signal representing the communication frame, for transmission over the communication channel.   
     
     
         4 . The transmitter for an OTFS transmission system according to  claim 3 , wherein the transmitter is arranged to assign between 50% and 99% of a total transmit power to data symbols, and a remaining transmit power to pilot symbols, preferably between 90% and 99%. 
     
     
         5 . The transmitter for an OTFS transmission system according to  claim 3 , wherein the transmitter is arranged to adapt a power assigned to data and pilot symbols, respectively, in dependence of a communication channel used, a carrier frequency used and/or a differential speed between the transmitter and the receiver. 
     
     
         6 . The transmitter for an OTFS transmission system according to  claim 3 , wherein the signal mapper is arranged to adapt the pilot overhead in dependence of a communication channel used, a carrier frequency used and/or a differential speed between the transmitter and the receiver. 
     
     
         7 . The receiver for an OTFS transmission system comprising a first receiver-side transformation unit and a second receiver-side transformation unit, wherein the receiver is adapted to receive a time-domain signal representing a communication frame in accordance with  claim 1  transmitted over a communication channel at an input of the first receiver-side transformation unit, which outputs a two-dimensional representation of the received communication frame in a time-frequency domain, and wherein the output of the first receiver-side transformation unit is provided to an input of the second receiver-side transformation unit, which outputs a two-dimensional representation of the received communication frame comprising pilot and data signals in the delay-Doppler domain, wherein at least the pilot signals output from the second receiver-side transformation unit are provided to a first channel estimation unit, which outputs a first estimation of a time-domain channel matrix (Ĥ t   i=0 ), wherein the first estimation of the time-domain channel matrix (Ĥ t   i=0 ), as well as at least the data signals output from the second receiver-side transformation unit or the pilot and data signals output from the second receiver-side transformation unit, are provided to an equalizer unit, which outputs an estimated set of at least data signals, wherein the estimated set of at least data signals, as well as at least the pilot signals output from the second receiver-side transformation unit or the pilot and data signals output from the second receiver-side transformation unit, are provided to a second channel estimation unit, which outputs a second estimation of the time-domain channel matrix (Ĥ t   i≥1 ), wherein the output (Ĥ t   i≥1 ) of the second channel estimation unit, as well as at least the data signals output from the second receiver-side transformation unit or the pilot and data signals output from the second receiver-side transformation unit, are provided to the equalizer unit, which outputs a further estimated set of at least data signals, wherein the receiver is adapted to iteratively repeat the channel estimation in the second channel estimation unit and estimating an estimated set of at least data signals in the equalizer unit until a termination criterion is met. 
     
     
         8 . The receiver for an OTFS transmission system according to  claim 7 , wherein the first channel estimation unit is adapted to perform a channel estimation based on a basis expansion modelling of a first BEM order of a time-varying communication channel. 
     
     
         9 . The receiver for an OTFS transmission system according to  claim 7 , wherein the equalizer unit performs a message passing, a zero-forcing and/or a minimum mean square error equalization. 
     
     
         10 . The receiver for an OTFS transmission system according to  claim 7 , wherein the second channel estimation unit is adapted to perform a channel estimation based on a basis expansion modelling of a second BEM order of a time-varying communication channel. 
     
     
         11 . The receiver for an OTFS transmission system according to  claim 7 , further comprising a control unit that is adapted to receive information about an absolute speed and direction of the receiver over ground, an absolute speed and direction of a transmitter over ground and/or a relative speed between the receiver ( 300 ) and the transmitter, and is further adapted to determine a BEM order (Q S ), and/or is adapted to receive the BEM order (Q S ) used at the transmitter for composing the communication frame, and is adapted to pass the received information and/or the BEM order (Q S ) to the first and/or second channel estimation unit. 
     
     
         12 . A method of transmitting a binary data sequence over an OTFS communication channel, comprising:
 mapping, in a signal mapper, a binary data sequence into a two-dimensional communication frame in the delay-Doppler domain in accordance with  claim 1 ,   transforming, in a first transmitter-side transformation unit, the two-dimensional communication frame in the delay-Doppler domain into a two-dimensional arrangement of information symbols in a time-frequency domain,   transforming, in a second transmitter-side transformation unit, the two-dimensional arrangement of information symbols in the time-frequency domain into a continuous time-domain signal representing the communication frame, and   transmitting the continuous time-domain signal representing the communication frame over the communication channel.   
     
     
         13 . The method of  claim 12 , further comprising setting a power allocation ratio between data and pilot symbols between 0.5 and 0.99, preferably between 0.9 and 0.99. 
     
     
         14 . The method of  claim 12 , further comprising adapting a power allocation ratio between data and pilot symbols in dependence of a communication channel used, a carrier frequency used and/or a differential speed between i) at least one of the first transmitter-side and the second transmitter side, and ii) the receiver. 
     
     
         15 . The method of  claim 12 , further comprising adapting the pilot overhead in dependence of a communication channel used, a carrier frequency used and/or a differential speed between i) at least one of the first transmitter-side and the second transmitter side, and ii) the receiver. 
     
     
         16 . A method of receiving a binary data sequence over an OTFS communication channel susceptive to doubly-selective fading, comprising:
 receiving a continuous time-domain signal representing a communication frame in accordance with  claim 1  over the communication channel,   transforming, in a first receiver-side transformation unit, the continuous time-domain signal representing the communication frame into a two-dimensional arrangement of information symbols in a time-frequency domain that is available at an output of the first receiver-side transformation unit,   transforming, in a second receiver-side transformation unit, the two-dimensional arrangement of information symbols comprising pilot and data signals in the time-frequency domain into a two-dimensional communication frame comprising pilot signals and data signals in the delay-Doppler domain that is available at an output of the second receiver-side transformation unit, providing at least the pilot signals output from the second receiver-side transformation unit to a first channel estimation unit, for obtaining a first estimation of a time-domain channel matrix (Ĥ t   i=0 ) at an output of the first channel estimation unit,   providing the first estimation of the time-domain channel matrix (Ĥ t   i=0 ), as well as at least the data signals output from the second receiver-side transformation unit ( 306 ) or the pilot and data signals output from the second receiver-side transformation unit ( 306 ), to an equalizer unit, for obtaining an estimated set of at least data signals at an output of the equalizer unit,   estimating, in a second channel estimation unit, an estimation of the time-domain channel matrix (Ĥ t   i≥1 ), from the estimated set of at least data signals that are output from the equalizer unit, as well as from at least the pilot signals obtained after the second transformation in the second receiver-side transformation unit or the pilot and data signals output from the second receiver-side transformation unit,   providing the estimation of the time-domain channel matrix (Ĥ t   i≥1 ) available at an output of the second channel estimation unit, as well as at least the data signals obtained after the second transformation in the second receiver-side transformation unit or the pilot and data signals output from the second receiver-side transformation unit, to the equalizer unit, for obtaining a further estimated set of at least data signals, and   iteratively repeating estimating the time-domain channel matrix (Ĥ t   i≥1 ) in the second channel estimation unit and estimating sets of at least data signals in the equalizer unit until a termination criterion is met.   
     
     
         17 . The method of  claim 16 , wherein obtaining the first estimation of the time-domain channel matrix (Ĥ t   i=0 ) in the first channel estimation unit comprises performing a channel estimation based on a basis expansion modelling of a time-varying communication channel of a first BEM order. 
     
     
         18 . The method of  claim 16 , wherein obtaining estimations of the time-domain channel matrix (Ĥ t   i≥1 ) in the second channel estimation unit comprises performing a channel estimation based on a basis expansion modelling of a time-varying communication channel of a second BEM order. 
     
     
         19 . The method of  claim 16 , wherein obtaining an estimated set of at least data signals in the equalizer unit comprises subjecting at least the data signals obtained after the second transformation in the second receiver-side transformation unit to a message passing, a zero-forcing and/or a minimum mean square error equalization. 
     
     
         20 . The method of  claim 16 , further comprising:
 receiving, in a control unit, information about an absolute speed and direction of the receiver over ground, an absolute speed and direction of a transmitter over ground and/or a relative speed between the receiver and the transmitter, and/or the BEM order (Q S ) used at the transmitter for composing the communication frame,   determining the respective BEM order to be used in the first and/or in the second channel estimation unit, and   providing the respective determined BEM order to the first and/or to the second channel estimation unit.